The present invention relates generally to sensors for measuring the properties of a gaseous medium and, more specifically, to a damping gyrometer for measuring the properties, e.g., temperature, of a gaseous medium based on the measured damping effects that are principally caused by thermally induced viscosity changes.
Many recent experiments involving the study of the interaction of weakly ionized gases and moving bodies have yielded anomalous observations that are not explained satisfactorily according to currently understood phenomena. The fractional degree of ionization needed to see the effects are on the order of 10xe2x88x926. Most of these effects are easiest to observe under highly energetic flow conditions. The most striking and potentially the most valuable effect concerns measurable changes in shock profiles, shock velocities and shock intensities.
Modified shockwaves can alter the way that an air vehicle interacts with its environment. In a dynamic flow setting where a vehicle surface is interacting with the weakly ionized gas, these effects can be translated into drag and surface heating rate variations that potentially afford an important mechanism for solid-state electronic control. The long-range goal of the study of these interactions is to understand the processes sufficiently to be able to apply them to optimizing the systems-level performance of aerodynamic vehicles.
Although the most striking measurable effects are seen under highly energetic flow conditions, these conditions are inherently difficult to study when trying to understand the basic physical processes that underlie the observed system behavior. The conditions are highly unsteady and measurements require the use of high-speed data acquisition systems. In addition, the electronic mechanisms for producing the weakly ionized gas can introduce excessive noise into the measurement system.
On a systems level, there is a price to be paid for the observed aerodynamic effects in terms of the input power needed to alter the medium. Some portion of the observed effects can often be attributed to the natural Joule-heating of the medium. It is necessary to separate the heating effects from xe2x80x9cotherxe2x80x9d energy exchange mechanisms to understand if there is any novel aspect that is attributable to the ionization. Possible xe2x80x9cotherxe2x80x9d mechanisms include non-equilibrium energy exchange between translational, vibrational, rotational and electronic states, storage of potential energy within transient plasma structures or electric double layers, and ion-acoustic interactions. To reduce the number of variables, a shock tube apparatus was used to study the processes associated with propagating shocks in a weakly ionized gas in a manner that was largely independent of boundary layer interactions.
In an experimental setup, the shock tube consisted of a high-pressure driver section and low-pressure section separated by a diaphragm. Gas was supplied through a bottle system. Air was usually used as the source gas but in some tests, carbon monoxide (CO) was also used to seed the gas for spectral signatures. Within the low-pressure section, copper anodes and nickel cathodes, mounted on the upper and lower surfaces, were used to create a uniform electrical discharge that was transverse to the shockwave propagation direction. The maximum available current was four amperes provided in constant current mode allowing current densities up to xcx9c150 A/m2.
Two laser beams were passed through the driven section and onto photodetectors to detect the passage of the shockwave after the diaphragm was burst. The photodetectors were thus able to measure both velocity and intensity of propagating shocks.
A previously reported main conclusion from the shock tube measurements was that the shock propagation velocity increased in proportion to the current density, a result that was not inconsistent with purely thermal effects. (See, Van Wie D. M., Wesner, A. L., and Gauthier, L. R., Jr., xe2x80x9cShock Wave Characteristics Measured in Gas Discharges,xe2x80x9d AIAA Paper 99-4824, November 1999.) However, since the gas temperature was difficult to measure during the discharge, other possible effects could not be ruled out.
To further resolve the effects, new devices and methods were needed to characterize the weakly ionized plasma and the unionized gas in the absence of a propagating shock.
The damping gyrometer of the invention can be used to measure minute forces exerted on a rotating body placed within the shock tube medium. In one embodiment, the damping gyrometer comprises at least two and, preferably, four rotating disks or paddles symmetrically mounted to a common central elevated low-friction pivot point via rising radial arms. The pivot point is the single point of mechanical support for the rotating apparatus/means. A stand with a concave glass element/lens provides a low-friction support as a pivot point seat for the pivot point. All elements of the apparatus are non-conductive.
Once set into motion, the only force acting on the gyrometer are the pivot point friction and the damping effects of the medium in which it spins. A laser beam and photodetector (or alternatively a laser displacement sensor), along with customized software algorithms are used to measure the rotational rate and, hence, importantly, the rotational rate of change overtime or deceleration rate of the apparatus. The deceleration of the damping gyrometer can then be used to determine properties of the medium in which it spins, including changes in density, pressure, and temperature. The measurement can also be directly related to the electron density in the case of weakly ionized gases.
In the present embodiment, short air blasts through a hollow tube are used to impart momentum to the rotating part of the damping gyrometer and acquired data are post-processed and interpreted offline. In another embodiment, the same laser used to determine rotational rate by extracting the paddle location measurements can be used to impart momentum to the rotating means. A microprocessor can be used to generate the time/frequency data and to calculate the parameters of the medium that are being measured. In this embodiment the rotating apparatus is placed within a chamber holding the medium of interest and can be set in motion and non-contact measurements taken through a window into the chamber by purely optical means.